If I am in F1 visa and willing to apply for Green card, during the process, can i still maintain my F1 visa?

Yes this is doable but if new visa is needed for international travel you may have a problem with conflicting intent once you get to the I-140...

After the recent policy change where individuals on F1 STEM OPT cannot work on client sites, are meetings at client site okay?

It is not clear this is strictly prohibited -- see various attorney analyses on this issue.

Should my son, 14 years old, apply for I-751 even though his temporary visa expires in 2020?

If he entered at a later date than you (more than 90 days later) then his I-751 must be filed separately.

Can my co-signer replace tax return transcript by form 1040?

I would not try to interfile this just file at the interview.

Withdraw I 485 based on I 140 and file I 485 again for the other category

You do not need to withdraw your employment -based I-485. Just file the one-step through you marriage including I-485/I-130?i-765/ and I-131. At...

Your passport would suffice if that is the name you want to use. If not, get a passort changed or accept the birth name until you legally get it...

Travel out of country for h4 Visa stamping ,while I-539 and H4 EAD application are pending?

Once she travels her change of status to H-4 will be considered abandoned. If she can link up her H status after her reentry before the H-4 EAD is...

Hard to answer without seeing the RFE. Are they claiming you are not eligible to work for Company B because company A's Petition was revoked for...

You are allowed to have more than one visa but can only be in one status at a time.

Emergency question---->Medical situation with 1 month visa left over

You can apply for a change of status to B-2 with proof of medical situation and that you have your own funds without public benefits to pay for the...